Artificial dental crowns cover portions of a tooth surface and are normally fabricated away from the patient's mouth, in a lab, and then installed in the mouth. Full crowns form the entire occlusal surface of a tooth and the sides to the gum line. Partial crowns include onlays which cover the occlusal surface and sometimes portions of the sides, inlays which mainly cover central portions of the occlusal surface and may extend partially onto the sides, and veneers which mainly cover only a side surface of a tooth. For simplicity, all of these artificial elements for covering tooth surfaces will often be referred to as crowns in this document, but it should be recognized that each procedure discussed herein does not necessarily apply to all artificial crowns.
In the field of fixed prosthodontics, the proper placement and adjustment of artificial crowns has always been a cumbersome process at best because it has necessitated placing several fingers of a dentist's hand within the patient's mouth during the process. This is because as practiced now, the placement and fitting of an artificial tooth crown normally entails the dentist holding onto the crown with fingers of one hand, and then manipulating the crown in the patient's mouth to place, mark, fit and cement the crown. Often, the marking, fitting and cementing requires fingers of the other hand to be placed into the patient's mouth as well.
Often an artificial crown is fashioned with the interproximal contacts somewhat tighter than necessary and then fitted by grinding and polishing it so that it will fit in proper contact with the adjacent teeth. Even if a crown is not made intentionally in the oversized manner, the crown may fit too tightly or too loosely because of accumulative errors due to impression materials and techniques, errors in stone dies and molds, changes in the dimension of the wax patterns, investing, casting, changes during porcelain firing, and other factors. Other causes of improper contacts are because the crown was incorrectly manufactured, or because the patient's teeth have moved slightly after the impression for the crown was made.
The interproximal contacts between an artificial crown must not be too tight or the adjacent teeth, which are attached flexibly to the jaw bone by the periodontal ligament, may spread apart and move out of alignment with the other teeth, including the artificially crowned tooth. Further, too tight of an interproximal contact may prevent the patient from being able to floss between the crowned tooth and the adjacent teeth, which could lead to gum disease and tooth decay. The contact fit must also not be too loose or the ensuing space between the artificially crowned tooth and the adjacent teeth will allow food and particulate matter to collect. The ideal fit is somewhere between a tight and loose fit, with light contact between the artificial crown or inlay and the surrounding teeth desired.
Many dentists use only dental floss to check whether interproximal crown contacts are too loose or too tight. This procedure is not accurate because of the thickness of floss, which is quite gross at best. The floss may also wedge the teeth apart and make it appear that the contact is proper.
Another method of checking the interproximal contact is by placing a non-marking thin plastic strip between the interproximal contact of the artificial crown and adjacent tooth and then seating the crown and pulling on the plastic ribbon to determine if the contact is too loose or too tight. This method is more precise than using floss, but is still very subjective, difficult to accomplish, and does not mark the contact point. This method also does not tell the operator precisely how strong the pressure is on each side of the crown.
Currently, the best method for checking the interproximal crown contact involves adjusting the contact fit of the artificial crown before its final cementing. This is done by holding the crown with two fingers, one on each side, reaching into the patient's mouth, and pressing the artificial crown onto the prepared tooth. While the crown is being placed on the tooth, a piece of thin marking ribbon is slipped with tweezers between the artificial crown and the adjacent tooth in front or behind the crown being placed. The dentist seats the crown fully onto the prepared tooth and pulls on the ribbon to remove it. An estimation of the tightness of the interproximal fit is made by estimating the pulling force necessary to remove the ribbon. This estimation is very subjective and not very accurate. Further, if the contact is too great, the ribbon will often tear when being removed. When the marking ribbon is removed, it will leave a small residue on the crown at any contact point between the adjacent tooth and the artificial crown being placed.
The artificial crown is then removed with the fingers or some dental tool such as extraction forceps or a sharp probe placed under a finish line margin of the crown. Use of these tools risks great damage to the crown. After the crown has been removed it can be ground and polished down at any heavy contact point for a more precise fit. As stated above, when using this process to estimate contact, the dentist must estimate the contact pressure. The dentist may therefore grind or polish the crown incorrectly on one or more of the sides, resulting in a finished crown which still fits improperly. This fitting method is problematic in that if there is excessive crown contact on one side of the crown only, the tooth to be crowned will shift and make the crown also contact the tooth on the opposite side. The dentist cannot always tell that this is occurring and will then grind both sides of the crown, only to find that when the large side is ground down and the tooth moves back, that the other side of the crown is no longer in contact.
If the crown is too small and there is a lack of contact, the dentist must send the crown back to the laboratory to have its size increased. The lab technician does not know how much too small the crown was and will thus greatly oversize the crown. The dentist will then later have to spend significant time resizing the crown.
This process of placing, checking, and adjusting is done repeatedly until the crown or inlay is fitted to the best of the dentist's abilities. The internal fit of the crown is then checked using a standard indicator paste or powder. Once the internal fit has been adjusted and deemed satisfactory, the occluding (biting) surface is then checked and adjusted. The crown is then cemented permanently onto the prepared tooth.
Patients often experience discomfort resulting from the introduction of multiple fingers into their mouth during the process of fitting an artificial crown. The dentist often has two fingers of one hand holding the crown and two fingers of the other hand holding and placing the ribbon, all in the mouth of the patient at the same time. Further, the fitting process is made difficult because of the tight quarters and the necessity of having so many fingers in the patient's mouth. The dentist not only has a difficult time maneuvering the crown and ribbon into place, especially when the tooth being crowned is in the rear of the mouth, but the dentist's view of the mouth is often blocked. Tweezers have been used to hold the testing ribbons, however, the tweezer jaws are at the wrong angle and are not designed for holding the ribbon flat. Further, tweezers do not readily aid the dentist in estimating the contact force when pulling the ribbon from the teeth.
The process of marking an artificial crown has been further complicated by the fact that past testing ribbons have not been designed for easy use. Previously, ribbons of paper or cloth have been used, but these are very thick and difficult to fit in between the adjacent teeth and the artificial crown being placed. Paper and cloth ribbons also loose their integrity when they encounter saliva. Further, paper, cloth, and even thin-plastic ribbons often come in large sheets or rolls from which the dentist or assistant must cut or tear small individual pieces, and then grip with fingers or tweezers for placement in the mouth.
These ribbon designs are very wasteful and cumbersome. When a dentist or assistant must hold a large ribbon sheet or roll and tear or cut it, the carbon or dye on the ribbon often becomes smeared and the ribbon crumpled. The possibility of contaminating the entire sheet or roll during the cutting and marking process often dictates that the remaining ribbon on the entire sheet or roll discarded. Hand cutting of the ribbon also leads to inconsistently sized ribbons. Further, because of electrostatic forces, small pieces of the cut ribbon often cling to each other, and gloves and tools, making them difficult to handle. Lastly, cut ribbons, especially those from rolls, often tend to curl, making them difficult to place.
Another problem which arises when the crowns are fitted by hand is contamination of the crown cement by substances on the dentist's latex gloves. Materials on the gloves such as zinc stearate, talc, corn starch and other substances which the manufacturers place on the gloves to keep them from sticking to each other act as contaminants in the cement.
Further, phosphoric acid and other chemicals such as eugenol in the cementing agent can dissolve or weaken latex gloves which come into contact with them. This creates a greater risk that a breach in the gloves will occur, possibly resulting in exposure of contaminants between the dentist and patient.
Lastly, the placement of an artificial crown for cementing or fitting has been tried in another fashion. Sometimes wax, clay, or clay-like materials have been used to adhere the artificial crown to a wooden, plastic or metal stick. The artificial crown is then set onto the tooth as directed by the stick. This method is troublesome, because the wax or clay material is often very soft and not always very sticky, which makes it difficult to adjust, place, and release the crown. Often the crown prematurely releases from the material. The clay or wax material is also a contaminant and may leave deposits which can get on the internal surfaces of the crown and interfere with subsequent placements or the permanent cementing of the artificial crown.
There is a need for an effective, yet less cumbersome, means for placing and fitting artificial tooth crowns in a patient's mouth. There is also a need for testing ribbon which can be easily accessed with little waste or contamination. Lastly, there is a need for a marking, fitting and placing method which will not contaminate the artificial tooth crown or inlay cementing process.